This invention relates to devices used in microwave systems, and more particularly to a circuit for normalizing the operating characteristic of devices tuned by a controlled current to permit devices to be interchanged without requiring realignment in the systems. There are at least two general classes of such devices: yttrium-iron-garnet (YIG) devices and traveling-wave maser (TWM) devices.
YIG devices are small solid-state devices that contain ytrrium-iron-garnet crystals used in combination with a controlled magnetic field for tuning. Such devices, which have been described in the literature, are used as oscillators, filters or harmonic generators. A single-crystal YIG sphere in a magnetic field is used as the tuning element in such devices because of the YIG's high "Q" resonance characteristic over wide frequency ranges. The technique for tuning the YIG device involves application of an external DC magnetic field which interacts with magnetic dipoles created by spinning electrons in the YIG sphere to cause them to align themselves in the direction of the field. Any magnetic force at right angles to the DC magnetic field causes the dipoles to precess about the fixed direction of the DC magnetic field. Since the rate of precession depends upon the strength of the net magnetic field, it is possible to tune the YIG device over a wide range by control of the field, and to then modulate the magnetic force at right angles to the DC magnetic field created in a small air gap between poles of a high permeability material in a pair of coaxial tuning coils. The problem with tuning YIG devices is that different devices have different current-frequency characteristics. Consequently, once a device is tuned for operation in a system, it can not be replaced with another device without realignment of the system.
The goal of interchangeability of YIG devices in microwave systems is important to facilitate maintenance of the system by simply substituting functional units containing YIG devices, or simply substituting YIG devices, without any realignment. The unit to unit variability of YIG-device performance in the system is often beyond the range which can be tolerated for reliable system operation, thus requiring realignment unless the replacement unit is so configured that it includes all of its drive circuits so that it can be prealigned for the system. It is not always feasible to so configure the units for replacement. To assure interchangeability, it is desirable to add to each interchangeable unit (YIG device) a circuit which normalizes the YIG device to some predetermined current-frequency characteristic.
TWM devices, which have been described in the literature, are very similar to YIG devices in that a DC magnetic field is required for maser operation, and the strength of the DC magnetic field must be adjusted for tuning the center frequency, gain and bandwidth. In some applications, an additional set of coils may be distributed along the length of the TWM structure for stagger tuning in order to shape the frequency response characteristic of the device. It may be desirable to normalize TWM devices in the same manner as YIG devices. Still other types of devices which rely on a controlled current to define its operating characteristics may be normalized by a circuit in the same manner.